The present invention relates generally to image scanners for providing a digitized image of a document, and in particular, to a rotary drum scanner for providing a digitized image from a transparent or reflective document.
Image scanners are utilized for digitizing both text and image sources. The image sources scanned include conventional documents such as letters, memoranda, photographs, transparencies, and the like. The two principle types of image scanners are flat bed scanners and rotary scanners. Flat bed scanners are constructed so that the image source is mountable on a flat surface or "bed." The image source is scanned from side to side and line by line and then processed into a desired digital image which is in a form suitable for high-speed processing. A problem with using flat-bed scanners is the complexity of the scanner which requires careful adjustment in order to maintain image quality. In addition, the resolution and the dynamic range of the digital image is limited because of the nature of the linear array used for scanning the image. Consequently, flat-bed image scanners are used in applications which do not require a high resolution or a wide dynamic range because flat-bed scanners have traditionally been priced at the low end of the image scanner price range.
A rotary scanner utilizes a rotary drum to mount the image source. The rotary scanner rotates the drum and linearly positions the drum to permit a light source to illuminate portions of the image source. An optical processing unit of the image scanner contains a scanning device which scans the illuminated portions of the image to form the digitized image. Both the light source and the scanning device are fixed in position and the rotary drum is moved linearly and rotated relative to the light source and the scanning device. Because the relative motion between the drum and the scanning device is primarily rotational in nature, high scanning rates can be achieved. The high scanning rates produce digital images with high resolutions and increase dynamic ranges. Thus, rotary scanners are typically used in commercial operations. Nevertheless, the nature of the rotary scanner has typically made it more expensive than flat-bed scanners. Thus, there is desire for a rotary scanner which provides high quality resolution and broad dynamic range at a lower cost than existing rotary scanners.
One problem associated with rotary scanners is that removing the drum to place a new image source on the drum has traditionally required complex alignment procedures. A quick release mechanism eases the removal of the drum. The Boston U.S. Pat. No. 4,024,518 discloses a rotary image scanner which includes such a quick release mechanism. The Boston patent mounts the drum between a headstock assembly and a tailstock assembly. The tailstock assembly is movably mounted on a carrier rail for axial movement towards the headstock assembly. The tailstock assembly has a spring loaded, conically tapered moving center element corresponding to a conical taper on the drum. Axial movement of the tailstock assembly is controlled by an actuator assembly where a handle of the actuator assembly is moved in a direction to move the tailstock assembly in an axial direction towards the headstock assembly. The moveable center element of the tailstock engages the left-hand conical drum aperture and moves the drum to the right until the right-hand tapered drum aperture is seated on the drum spindle. In this way, the drum is actually aligned with a rotational axis of the drive spindle.
Another type of quick release mechanism is used in the Howtek, Inc. Scan Master D4000 image scanner. Again, the drum is mounted between a headstock and a tailstock. However, unlike the Boston patent, the headstock and tailstock are fixedly mounted to a first carriage and a second carriage respectfully. The first carriage is fixedly mounted on a base, and the second carriage is movably mounted on the base. The Howtek Scanner includes an arm mounted on one of the carriages and rotatable to force the carriages apart, which allows the drum to be inserted when the arm is in the open position to force the carriages apart. In addition, a spring inner-connects the carriages to bias the carriages towards each other when the arm is in the closed position so that the drum is secured between the headstock and the tailstock.